1. Field of the Invention
The present invention relates, in general, to a hydrogen generator and, more particularly, to a natural gas-using hydrogen generator, which can produce high purity hydrogen from natural gas itself by reacting natural gas with water without the aid of additional equipment.
2. Description of the Prior Art
Production of hydrogen from natural gas is generally accomplished by three techniques: steam reforming; partial oxidation; and autothermal reforming.
Of them, the partial oxidation and the autothermal reforming techniques are economically unfavorable owing to the limit that additional oxygen is to be fed. Thus, the steam reforming technique has been widely employed. Here, the following description will be given based on steam reforming technique.
A typical steam reforming process consists mainly of a reforming step where hydrocarbons are reacted with heated steam to produce various reformed gases and a refining and recovering step where hydrogen gas is recovered from the reformed gases. Examples of the hydrocarbons include natural gas, propane, butane, naphtha, etc.
The following is a detailed description for a conventional steam reforming technique using natural gas as a raw material. Natural gas includes methane as its major component, which reacts with steam as follows: EQU CH.sub.4 +H.sub.2 O=3H.sub.2 +CO H=206 kJ/gmol (I)
The production of hydrogen and carbon monoxide from natural gas and steam according to Reaction I is usually carried out in the presence of a modifying catalyst (e.g. Ni). As shown, this reaction is strongly endothermic, so that an external heat should be supplied. For this reaction, a temperature of 500 to 1,000.degree. C. should be maintained under a pressure of 1 to 20 atm. To prevent the reverse reaction of methane production and the production of coke on a catalyst, excess steam is to be fed into the reaction system. In case of natural gas, the mole ratio of steam to hydrocarbon should be maintained in a range of about 3 to 3.5.
In a large hydrogen plant, a reformer comprises a large furnace in which a plurality of catalyst tubes are operated at a temperature of 900 to 1,000.degree. C. In spite of such high temperatures, its heat efficiency is as low as 60 to 70%.
In addition to Reaction I, the conversion reaction of carbon monoxide into hydrogen occurs as represented by the following Reaction II: EQU CO+H.sub.2 O=H.sub.2 +CO.sub.2 H=-41 kJ/gmol (II)
Reaction II, so-called water/gas shift reaction, is exothermic so that, as the reaction temperature is lower, the conversion of carbon monoxide is higher. Accordingly, the reaction is heated only up to a temperature of 180 to 300.degree. C. while maintaining the pressure from 1 to 20 atm. Carbon monoxide is high in concentration at the outlet of the reformer, which is operated at a high temperature. A heat exchanger is provided at the tail of the reformer with the aim of lowering the temperature of the gas and the gas is allowed to pass through a conversion reactor (in which Cu type catalyst is filled) with the aim of lowering the concentration of carbon monoxide and increasing the yield of hydrogen.
Natural gas contains a trace of sulfur compounds which serve to alert the leakage of natural gas by their characteristic odor. During the reaction, these sulfur compounds are coated on the reformer catalysts, which then become incapacitated. Thus, it should be pre-treated to be desulfurized. For this, a desulfurization reactor filled with hydrodesulfurization catalysts and absorption catalysts of H.sub.2 S (hydrogen sulfide) is set at the head of the reformer and maintains a temperature of 150 to 350.degree. C.
Largely, the start up time of the total system depends upon the period which it takes the desulfurization reactor to be ready for operation. Conventionally, the desulfurization reactor is warmed-up by an electric heater or indirectly preheated by exchanging the heat which occurs when nitrogen flows. In addition, an additional gas boiler is set to convert water material into steam.
This electric heater which is to warm up the desulfurization reactor of the conventional natural gas-using hydrogen generator causes a high temperature locally around it, deteriorating the desulfurization catalyst. In addition, the electric heater is economically unfavorable because the maintenance cost is high owing to its consuming a great deal of power. Because the electric heater is difficult to set up in the desulfurization reactor, its assembly is significantly low in productivity.
Further, the additional gas boiler is required to generate steam, which results in an excess of the hydrogen generator. Moreover, the product gas containing hydrogen from the reformer does not reach the conversion reactor without heat recovery and reuse, so that the total heat efficiency of the hydrogen generator is remarkably lowered.